Part of it is also simply because it is a documentary........ that's sort of a strict "no interference" thing. They're trying to show the life history of an animal in all its sometimes-gory detail. There wouldn't be much point to documentaries if every other scene was "and here's some more people going in and saving the animal from nature".

BTW, I feel the same way when watching cheetah documentaries where cubs get killed by lions - cheetahs have a hard enough time even breeding anymore, I can't imagine just sittting there while they get killed by another predator for no good reason.